Overhead projectors are in common use for enlarging images from transparencies while projecting the images onto a viewing surface. Typically, such transparencies are approximately 21.6 by 28 centimeters and the projectors include a platen somewhat larger in size for supporting the transparencies over a light source. A fresnel lens condenses illumination from the source and directs it through the platen and transparency to converge 25 to 30 centimeters above the center of the platen. Optics of similar focal length are located there for enlarging the image and redirecting it horizontally toward a viewing surface. The degree of enlargement depends on the focal length of the optics and the image distance between the optics and the viewing surface.
Photographic slides are another popular medium for recording presentation material. Such slides are much smaller than the transparencies described above, having an image area approximately 43 millimeters on the diagonal, and traditionally are used in a projector having a more concentrated light source and smaller diameter optics of shorter focal length. A 12 to 16 centimeters focal length, for example, provides greater enlargement than a typical overhead projector at the same image distance.